It’s called cryptozoology, the study of creatures once thought to have been mythical or even extinct — and then found

Chacoan peccary

When, in 1974, the American biology professor Ralph Wetzel discovered a “rangy
big pig” in Paraguay, he was astounded. This 45kg relative of pigs, boars
and warthogs was known from fossils but was thought to have become extinct
10,000 years ago. Not so. It is the largest of the peccary species but,
unlike its cousins, has a third hind toe. Like many “rediscovered” species,
it is in danger of becoming extinct again, a victim of hunting and habitat
destruction.

Pygmy elephant

This “mini” elephant was discovered, and then described as a separate species
(Loxodonta pumilio), by the German